It is easier than ever to buy and sell used designer clothing, with the growth of resale sites such as Poshmark, ThredUP, Tradesy, The RealReal, Vestiaire Collective and others. But for women seeking to clear out their closets—or looking to buy a gently used handbag—it is hard to tell when the price is right. Kelley Blue Book will estimate the value of your Chevy, but for fashion, it is guess as guess can.